Computers have traditionally used cathode ray tubes (CRTs) for displaying information. CRTs are inherently analog in nature, and thus traditional video interfaces use analog signals to convey display data from a computer system to a display device. A popular standard is the Video Electronics Standards Association (VESA) RGB standard for analog display devices. VESA compliant analog monitors support VESA Display Data Channel (DDC) protocol for communicating the monitor functionality to a graphics control card.
With the increasing popularity of flat panel display (FPD) technologies, (such as, for example liquid crystal displays (LCDs)), digital video interface standards have been developed. One such standard is the Digital Visual Interface (DVI) developed by the Digital Display Working Group (DDWG), a consortium of computer industry manufacturers. The DVI standard provides a high speed digital connection for display monitors that is independent of display technology. This standard provides for “plug and play” functionality, enabling a host system to automatically determine the capabilities of a monitor connected to it, and then properly format information for display on the monitor. As is well known in the art, the DVI specification currently supports two types of physical connectors: DVI-Digital (DVI-D) and DVI-Integrated (DVI-I).
The DVI-D connector supports a single digital display, and has 24 pins organized in three rows of eight contacts. The signal pin assignments are listed in the table of FIG. 1. The DVI-D connector can accommodate one or two digital links driven by Transition Minimized Digital Signaling (T.M.D.S.), which provides DC balanced signals. These digital links are used to transmit graphics data to the display. With reference to the table of FIG. 1, a first digital link consists of Data channels 0, 1 and 2 (“T.M.D.S. Data0−”, “T.M.D.S. Data0+”, “T.M.D.S. Data1−”, “T.M.D.S. Data1+”, “T.M.D.S. Data2−”, “T.M.D.S. Data2+”, (pins 17, 18, 9, 10, 1, 2)) as well as “T.M.D.S. Clock-” and “T.M.D.S. Clock+” (pins 24, 23). Plug and Play functionality is provided by a DDC signaling channel: “DDC Clock” and “DDC Data” (pins 6, 7) with the associated “+5V” and ground (pins 14, 15). For high resolution displays (especially high pixel format digital CRTs), the second T.M.D.S. link can be used to augment the bandwidth of the first link. The second digital link consists of Data channels 3, 4 and 5 (“T.M.D.S. Data3−”, “T.M.D.S. Data3+”, “T.M.D.S. Data4−”, “T.M.D.S. Data4+”, “T.M.D.S. Data5−”, “T.M.D.S. Data5+”, (pins 12, 13, 4, 5, 20, 21)) The two T.M.D.S. links share the same clock and DDC channel.
For the purposes of the present invention, a “channel” is defined as a single path for conveying electrical signals. For example, the T.M.D.S. “channel O” is a path for conveying digital blue video information. This channel consists of a balanced pair of conductors “T.M.D.S. Data0−” (pin 17) and “T.M.D.S. Data0+” (pin 18). By contrast, the “DDC Data” channel uses a single conductor (pin 7) referenced to ground. Therefore a “channel” can represent one or more conductors for conveying one signal.
The DVI-I connector has two digital links as in the DVI-D connector and in addition, an analog link, and can support a digital display, or an analog display. This connector has 29 pins that are divided into two sections. The signal pin assignments are listed in the table of FIG. 2. The first section is organized as three rows of eight contacts and is compatible with DVI-D. The second section of the DVI-I connector contains five contacts that are designed specifically for analog video signals. With reference to the table of FIG. 2, the analog link consists of pins C1 (“Analog Red”), C2 (“Analog Green”), C3 (“Analog Blue”), C4 (“Analog Horizontal Sync”), C5 (“Analog Ground”) and 8 (“Analog Vertical Sync”). Pins 6 (“DDC Clock”), 7 (“DDC Data”), 14 (“+5V”) and 15 (“Ground”) are used by either the digital monitor or the analog monitor, which ever is connected.
Thus, a single DVI-I connector can support current and future digital display devices as well as maintain compatibility with legacy analog displays as follows:                A DVI compatible display device can be connected to the DVI-I system-side connector via either a DVI-D or DVI-I connector, or        An RGB (VESA Video Signal Standard) analog display device can be connected to the DVI-I system-side connector using a DVI-I connector.        
A limitation of the DVI-I standard interface is that dual head operation is not supported for analog displays. Thus, even when a graphics card has sufficient bandwidth to support two displays, the DVI-I interface limits the usable bandwidth, for analog displays, to a single display.
Accordingly, a system for providing analog dual-head support using a single DVI-I connector remains highly desirable.